This invention relates generally to the separation of aliphatically unsaturated hydrocarbons from hydrocarbon mixtures. More particularly, the present invention relates to membranes suitable for the separation of olefins from paraffins.
The aliphatically unsaturated hydrocarbons, which typically are obtained in admixture with other hydrocarbons as the by-products of chemical syntheses or separations, are important reactive materials for preparing polymers and in other applications. While distillation of the unsaturated hydrocarbons from the streams in which they are found is feasible when the hydrocarbons are normally liquid or can readily be made so and the boiling points of the other feedstream components differ sufficiently, more expensive procedures, such as cryogenic distillation or extractive distillation, are required when the feedstream is gaseous at ambient conditions or the components of the feedstream are close-boiling. Thus there is considerable commercial motivation for developing alternative processes for separating aliphatically-unsaturated hydrocarbons from the hydrocarbon streams in which they are found.
The known property of aliphatically unsaturated hydrocarbons to complex reversibly with certain metals or metal ions, particularly transition metals such as silver and the salts thereof (see, e.g., Chatt, J., "Cationic Polymerization and Related Complexes"--The General Chemistry of Olefin Complexes with Metallic Salts, pp. 40-56, Proc. Conf. Univ. Coll. North Saffordshire, England, 1952), has been the basis for various processes for purifying unsaturated hydrocarbons.
U.S. Pat. No. 2,685,607, for example, proposes separating olefins from paraffins by contacting with silica gel impregnated with aqueous silver nitrate. U.S. Pat. No. 2,458,067 employs a solution of silver in acetonitrile as an extractant. See also U.S. Pat. No. 3,395,192. U.S. Pat. No. 4,174,353 discloses a process for separating ethylene or propylene from a hydrocarbon cracking stream by extracting the olefins into an aqueous silver salt solution.
Reversible complexing agents in association with a membrane support have been employed for "facilitated transport" of unsaturated hydrocarbons through the membrane to achieve purification or separation of the unsaturated hydrocarbons from mixtures thereof. See, for example, Ward III et al., Science, 156, 1481 (1967); Way et al., J. Mem. Sci., 12, 239 (1982); Way et al., AIChE J., 33, 480 (1987) and Way et al., SRI International, Research Brief (May 15, 1987).
The metal complexing agent is selected so that the complex of metal and aliphatically unsaturated hydrocarbon forms readily and also readily reverts to its separate constituents under the conditions which exist on the permeate side of the membrane. The released unsaturated hydrocarbons having permeated the membrane are removed from its vicinity by suitable means such as by a sweep gas or through the effect of vacuum. While in the absence of the complexing metal there may occur some slight separation of feed components due to differing permeabilities across the membrane support, the presence of the metal complexes in association with the membrane provides enhanced selectivity for the unsaturated hydrocarbons.
U.S. Pat. No. 3,773,844 to Perry (assigned to Monsanto Company) discloses a membrane pervaporation process for separating mono-alkenes from hydrocarbon mixtures. The polymeric membrane contains a transition metal such as silver molecularly dispersed therein, the metal preferably being in an oxidation state to permit chemical interaction between it and the monoalkene, and also preferably chemically interacting with the polymer material in which it is dispersed, to minimize metal loss. Suitable polymers are indicated to include polyacrylonitrile, polyvinyl alcohol, polyvinylchloride, cellulose, cellulose esters, nylon, polyethylene, polystyrene, neoprene, copolymers of acrylonitrile and styrene, and copolymers of acrylonitrile and other polymers. Preferred polymers are those which contain groups capable of forming covalent or ionic bonds with the metal: for covalent bonding, groups such as the amine, amide, nitrile, alcohol, carbonyl, ether, sulfur, or carbon groups (including groups which contain the carbon-carbon double bond) may be used; for ionic bonding, carboxylate, sulfonic, phosphonate, phosphonic, arsenic and telluric moieties or end groups are employed.
The membranes are formed by casting from a solution or dispersion of the polymer and a soluble form of the metal, or by melt pressing a mixture of the powdered polymer and metal.
The '844 patent further teaches that improved monoalkene permeation may be obtained by a "conditioning" of the membrane prior to use to effect the replacement of undesirable ligands (e.g. from the solvent) from the metal by ligands which are said to be more easily displaced during permeation, thus permitting greater interaction between the metal and the alkene. This "preconditioning" step comprises soaking the membrane in a solution containing the displacing ligand or by casting the polymer membrane from a solution which contains, in addition to the polymer and the metal species and solvent, an organic material which contains an alkene linkage (col. 6, 11. 22-32).
In the Examples of the '844 patent, mixtures of styrene and ethylbenzene, and hexene and hexane, are contacted under pervaporation conditions against various polymer membranes such as polyvinylchloride, acrylonitrile polymers or copolymers, an aromatic hydrazide-amide polymer and an ethylene/acrylic acid copolymer. It is not indicated whether any of the membranes was treated by preconditioning.
A separation factor of up to 7.22 was reported for the separation of hexene from hexane employing a membrane comprising a copolymer of acrylonitrile and vinylpyridine.
U.S. Pat. Nos. 3,758,603 to Steigelmann et al. and 3,758,605 to Hughes et al. (both assigned to Standard Oil Company) disclose a process for separating unsaturated hydrocarbons from gaseous mixtures employing liquid barrier permeation and metal complexing techniques.
A liquid barrier comprising an aqueous solution of a metal which reversibly complexes with the unsaturated hydrocarbon is placed in contact with a semi-permeable membrane which is permeable to the gas-phase hydrocarbon mixture.
The membrane serves to immobilize the liquid barrier adjacent to or within the feed side of the membrane. While in the absence of the immobilized liquid, essentially all of the gas-phase components of the feedstock may permeate the membrane, the physical passage of the vapors in the presence of such a barrier is reduced or prevented; and therefore in order to traverse the film, a component of the feed stream must become a part of and then separate from the liquid barrier phase. It is intended that there be little, if any, passage of the feed components across the membrane except by interaction with the liquid barrier, and thus the liquid barrier controls the selectivity of the liquid barrier semi-permeable membrane.
In Steigelmann et al. the membrane is said to be essentially impermeable to the liquid barrier which is placed in contact with it. Suitable membranes are indicated to comprise cellulose acetate, nylon, polyvinyl chloride, polyvinyl alcohols, olefin polymers such as polyethylene, polypropylene and ethylene-propylene copolymers.
In Hughes et al. wherein the liquid barrier is placed within a hydrophilic, semi-permeable membrane, suitable membranes are exemplified by the polyurethanes, such as are obtained by reaction of polyisocyanates with an aliphatic polyol such as polyvinyl alcohol, although other polymers may be used if made sufficiently hydrophilic by incorporation into the polymer of hygroscopic agents, such as polyvinyl alcohols, polyacrylic acids, polyvinyl ethers, poloxyalkylene glycols and their carboxylic acid esters, and like polymers; as well as non-polymeric hygroscopic agents such as ethylene glycol, glycerol and propylene glycol, and alkylated carboxycellulose derivatives.
See also related Standard Oil patents 3,864,418, 3,865,890, 3,940,469, 3,951,621, 3,980,605, 4,014,665, 4,060,566, 4,200,714, 4,235,983, 4,239,506.
U.S. Pat. No. 4,318,714 to Kimura et al. (General Electric Company) discloses a process for facilitated transport of gases by means of a humidified ion-exchange membrane containing mobile counterions within its pores which are said to be retained within the membrane surfaces by the requirement of maintaining electroneutrality. The membrane is said to actively take part in the facilitation of gas permeation rather than serving merely as a support for the immobilized liquid contained therein.
In the examples, the membrane was prepared by soaking an ion-exchange membrane in an aqueous solution containing the ion. In Example 4 the separation of olefins from a gas mixture is reported, using a sulfonated polyxylene oxide ion-exchange membrane cast from a solution of chloroform and methanol, prior to immersion in silver nitrate solution for conversion to the silver counter-ion form. The feed gases comprised either pure ethylene or pure ethane humidified to 90 percent relative humidity. Ethylene permeability at 25.degree. C. was indicated to be 230.times.10.sup.-9 cc cm/sec cm.sup.2 cm Hg, and the permeability of ethane under corresponding conditions was indicated to be 0.8.times.10.sup.-9 cc cm/sec cm.sup.2 cm Hg. An ethylene/ethane separation factor of about 300 was reported.
U.S. Pat. No. 4,614,524 to Kraus (Monsanto Company) describes a water-free immobilized liquid membrane for facilitated transport of aliphatically-unsaturated hydrocarbons. The membranes are hydrophilic, semi-permeable preformed polymeric membranes capable of chemically bonding positive metal ions, and being plasticized by treatment with polyhydric alcohols. The plasticization with polyhydric alcohols is said to obviate the need for water in the membrane itself or in the feed stream. Selectivities with respect to a dry ethylene/ethane mixture were reported as about 8 to 15 and permeabilities were from about 5 to 10.times.10.sup.-10 in water-free feedstreams under ambient conditions. Preformed membranes indicated to be suitable are halogenated polyolefins with pendant acid groups, sulfonated polymers, carboxylated polymers, polyacrylic acids, and the like.
See also Published UK Patent Application GB 2,169,301 of Kraus (also assigned to Monsanto) which discloses water-free facilitated transport membranes which comprise a separation barrier of metal ions in solution with one or more polyhydric alcohols, the barrier separation membrane being incorporated into the pores or on the surface of hydrophobilic or hydrophilic membrane materials.
However, commercial application of immobilized liquid membranes is hampered by such factors as the gradual leaching of the metal-containing solution from the membrane and the fragility of liquid membranes in the presence of a transmembrane pressure difference. Further, efforts to achieve satisfactory flux in separations processes employing such membranes have been hindered by the constraints on membrane thinness imposed by the necessity to provide adequate support for the liquid phase.
It has been found that membranes prepared from certain hydrophilic polymers containing metals capable of complexing with aliphatically unsaturated hydrocarbons provide permeability high (flux) and selectivity for unsaturated hydrocarbons.
It has been further found that when such polymers have been treated in the presence of a cross-linking agent under conditions effective to bring about cross-linking of the polymer, the resulting membranes provide high permeability (flux) and selectivity for unsaturated hydrocarbons, with improved stability, particularly in the presence of liquid water.
It has also been found that even further improved flux and selectivity of the membrane for unsaturated hydrocarbons are obtained when the cross-linked polymer membrane also contains a hydrophilic salt of a Group I metal.